This invention relates to electrode structures. These electrode structures are especially intended for use in electro-optic displays but may also find use in other applications.
The terms “bistable” and “bistability” are used herein in their conventional meaning in the art to refer to displays comprising display elements having first and second display states differing in at least one optical property, and such that after any given element has been driven, by means of an addressing pulse of finite duration, to assume either its first or second display state, after the addressing pulse has terminated, that state will persist for at least several times, for example at least four times, the minimum duration of the addressing pulse required to change the state of the display element. It is shown in U.S. Pat. No. 7,170,670 that some particle-based electrophoretic displays capable of gray scale are stable not only in their extreme black and white states but also in their intermediate gray states, and the same is true of some other types of electro-optic displays. This type of display is properly called “multi-stable” rather than bistable, although for convenience the term “bistable” may be used herein to cover both bistable and multi-stable displays.
The term “electro-optic”, as applied to a material or a display, is used herein in its conventional meaning in the imaging art to refer to a material having first and second display states differing in at least one optical property, the material being changed from its first to its second display state by application of an electric field to the material. Although the optical property is typically color perceptible to the human eye, it may be another optical property, such as optical transmission, reflectance, or luminescence or, in the case of displays intended for machine reading, pseudo-color in the sense of a change in reflectance of electromagnetic wavelengths outside the visible range.
Some electro-optic media are solid in the sense that the materials have solid external surfaces, although the media may, and often do, have internal liquid- or gas-filled spaces. Displays using solid electro-optic media may hereinafter for convenience be referred to as “solid electrophoretic displays”.
Several types of electro-optic displays are known. One type of electro-optic display is a rotating bichromal member type as described, for example, in U.S. Pat. Nos. 5,808,783; 5,777,782; 5,760,761; 6,054,071 6,055,091; 6,097,531; 6,128,124; 6,137,467; and 6,147,791 (although this type of display is often referred to as a “rotating bichromal ball” display, the term “rotating bichromal member” is preferred as more accurate since in some of the patents mentioned above the rotating members are not spherical). Such a display uses a large number of small bodies (typically spherical or cylindrical) which have two or more sections with differing optical characteristics, and an internal dipole. These bodies are suspended within liquid-filled vacuoles within a matrix, the vacuoles being filled with liquid so that the bodies are free to rotate. The appearance of the display is changed by applying an electric field thereto, thus rotating the bodies to various positions and varying which of the sections of the bodies is seen through a viewing surface. This type of electro-optic medium is typically bistable.
Another type of electro-optic display uses an electrochromic medium, for example an electrochromic medium in the form of a nanochromic film comprising an electrode formed at least in part from a semi-conducting metal oxide and a plurality of dye molecules capable of reversible color change attached to the electrode; see, for example O'Regan, B., et al., Nature 1991, 353, 737; and Wood, D., Information Display, 18(3), 24 (March 2002). See also Bach, U., et al., Adv. Mater., 2002, 14(11), 845. Nanochromic films of this type are also described, for example, in U.S. Pat. Nos. 6,301,038; 6,870,657; and 6,950,220. This type of medium is also typically bistable.
Another type of electro-optic display is an electro-wetting display developed by Philips and described in Hayes, R. A., et al., “Video-Speed Electronic Paper Based on Electrowetting”, Nature, 425, 383-385 (2003). It is shown in U.S. Pat. No. 7,420,549 that such electro-wetting displays can be made bistable.
One type of electro-optic display, which has been the subject of intense research and development for a number of years, is the particle-based electrophoretic display, in which a plurality of charged particles move through a fluid under the influence of an electric field. Electrophoretic displays can have attributes of good brightness and contrast, wide viewing angles, state bistability, and low power consumption when compared with liquid crystal displays. Nevertheless, problems with the long-term image quality of these displays have prevented their widespread usage. For example, particles that make up electrophoretic displays tend to settle, resulting in inadequate service-life for these displays.
As noted above, electrophoretic media require the presence of a fluid. In most prior art electrophoretic media, this fluid is a liquid, but electrophoretic media can be produced using gaseous fluids; see, for example, Kitamura, T., et al., “Electrical toner movement for electronic paper-like display”, IDW Japan, 2001, Paper HCS1-1, and Yamaguchi, Y., et al., “Toner display using insulative particles charged triboelectrically”, IDW Japan, 2001, Paper AMD4-4). See also U.S. Pat. Nos. 7,321,459 and 7,236,291. Such gas-based electrophoretic media appear to be susceptible to the same types of problems due to particle settling as liquid-based electrophoretic media, when the media are used in an orientation which permits such settling, for example in a sign where the medium is disposed in a vertical plane. Indeed, particle settling appears to be a more serious problem in gas-based electrophoretic media than in liquid-based ones, since the lower viscosity of gaseous suspending fluids as compared with liquid ones allows more rapid settling of the electrophoretic particles.
Numerous patents and applications assigned to or in the names of the Massachusetts Institute of Technology (MIT) and E Ink Corporation describe various technologies used in encapsulated electrophoretic and other electro-optic media. Such encapsulated media comprise numerous small capsules, each of which itself comprises an internal phase containing electrophoretically-mobile particles in a fluid medium, and a capsule wall surrounding the internal phase. Typically, the capsules are themselves held within a polymeric binder to form a coherent layer positioned between two electrodes. The technologies described in the these patents and applications include:                (a) Electrophoretic particles, fluids and fluid additives; see for example U.S. Pat. Nos. 7,002,728 and 7,679,814;        (b) Capsules, binders and encapsulation processes; see for example U.S. Pat. Nos. 6,922,276 and 7,411,719;        (c) Films and sub-assemblies containing electro-optic materials; see for example U.S. Pat. Nos. 6,982,178 and 7,839,564;        (d) Backplanes, adhesive layers and other auxiliary layers and methods used in displays; see for example U.S. Pat. Nos. D485,294; 6,124,851; 6,130,773; 6,177,921; 6,232,950; 6,252,564; 6,312,304; 6,312,971; 6,376,828; 6,392,786; 6,413,790; 6,422,687; 6,445,374; 6,480,182; 6,498,114; 6,506,438; 6,518,949; 6,521,489; 6,535,197; 6,545,291; 6,639,578; 6,657,772; 6,664,944; 6,680,725; 6,683,333; 6,724,519; 6,750,473; 6,816,147; 6,819,471; 6,825,068; 6,831,769; 6,842,167; 6,842,279; 6,842,657; 6,865,010; 6,967,640; 6,980,196; 7,012,735; 7,030,412; 7,075,703; 7,106,296; 7,110,163; 7,116,318; 7,148,128; 7,167,155; 7,173,752; 7,176,880; 7,190,008; 7,206,119; 7,223,672; 7,230,751; 7,256,766; 7,259,744; 7,280,094; 7,327,511; 7,349,148; 7,352,353; 7,365,394; 7,365,733; 7,382,363; 7,388,572; 7,442,587; 7,492,497; 7,535,624; 7,551,346; 7,554,712; 7,583,427; 7,598,173; 7,605,799; 7,636,191; 7,649,674; 7,667,886; 7,672,040; 7,688,497; 7,733,335; 7,785,988; 7,843,626; 7,859,637; 7,893,435; 7,898,717; 7,957,053; 7,986,450; 8,009,344; 8,027,081; 8,049,947; 8,077,141; 8,089,453; 8,208,193; and 8,373,211; and U.S. Patent Applications Publication Nos. 2002/0060321; 2004/0105036; 2005/0122306; 2005/0122563; 2007/0052757; 2007/0097489; 2007/0109219; 2007/0211002; 2009/0122389; 2009/0315044; 2010/0265239; 2011/0026101; 2011/0140744; 2011/0187683; 2011/0187689; 2011/0286082; 2011/0286086; 2011/0292319; 2011/0292493; 2011/0292494; 2011/0297309; 2011/0310459; and 2012/0182599; and International Application Publication No. WO 00/38000; European Patents Nos. 1,099,207 B1 and 1,145,072 B 1;        (e) Color formation and color adjustment; see for example U.S. Pat. Nos. 7,075,502 and 7,839,564;        (f) Methods for driving displays; see for example U.S. Pat. Nos. 7,012,600 and 7,453,445;        (g) Applications of displays; see for example U.S. Pat. Nos. 7,312,784 and 8,009,348; and        (h) Non-electrophoretic displays, as described in U.S. Pat. Nos. 6,241,921; 6,950,220; 7,420,549 and 8,319,759; and U.S. Patent Application Publication No. 2012/0293858.        
Many of the aforementioned patents and applications recognize that the walls surrounding the discrete microcapsules in an encapsulated electrophoretic medium could be replaced by a continuous phase, thus producing a so-called polymer-dispersed electrophoretic display, in which the electrophoretic medium comprises a plurality of discrete droplets of an electrophoretic fluid and a continuous phase of a polymeric material, and that the discrete droplets of electrophoretic fluid within such a polymer-dispersed electrophoretic display may be regarded as capsules or microcapsules even though no discrete capsule membrane is associated with each individual droplet; see for example, the aforementioned U.S. Pat. No. 6,866,760. Accordingly, for purposes of the present application, such polymer-dispersed electrophoretic media are regarded as sub-species of encapsulated electrophoretic media.
A related type of electrophoretic display is a so-called “microcell electrophoretic display”. In a microcell electrophoretic display, the charged particles and the fluid are not encapsulated within microcapsules but instead are retained within a plurality of cavities formed within a carrier medium, typically a polymeric film. See, for example, U.S. Pat. Nos. 6,672,921 and 6,788,449, both assigned to Sipix Imaging, Inc.
Although electrophoretic media are often opaque (since, for example, in many electrophoretic media, the particles substantially block transmission of visible light through the display) and operate in a reflective mode, many electrophoretic displays can be made to operate in a so-called “shutter mode” in which one display state is substantially opaque and one is light-transmissive. See, for example, U.S. Pat. Nos. 5,872,552; 6,130,774; 6,144,361; 6,172,798; 6,271,823; 6,225,971; and 6,184,856. Dielectrophoretic displays, which are similar to electrophoretic displays but rely upon variations in electric field strength, can operate in a similar mode; see U.S. Pat. No. 4,418,346. Other types of electro-optic displays may also be capable of operating in shutter mode. Electro-optic media operating in shutter mode may be useful in multi-layer structures for full color displays; in such structures, at least one layer adjacent the viewing surface of the display operates in shutter mode to expose or conceal a second layer more distant from the viewing surface.
An encapsulated electrophoretic display typically does not suffer from the clustering and settling failure mode of traditional electrophoretic devices and provides further advantages, such as the ability to print or coat the display on a wide variety of flexible and rigid substrates. (Use of the word “printing” is intended to include all forms of printing and coating, including, but without limitation: pre-metered coatings such as patch die coating, slot or extrusion coating, slide or cascade coating, curtain coating; roll coating such as knife over roll coating, forward and reverse roll coating; gravure coating; dip coating; spray coating; meniscus coating; spin coating; brush coating; air knife coating; silk screen printing processes; electrostatic printing processes; thermal printing processes; ink jet printing processes; electrophoretic deposition (See U.S. Pat. No. 7,339,715); and other similar techniques.) Thus, the resulting display can be flexible. Further, because the display medium can be printed (using a variety of methods), the display itself can be made inexpensively.
An electro-optic display normally comprises a layer of electro-optic material and at least two other layers disposed on opposed sides of the electro-optic material, one of these two layers being an electrode layer. In most such displays both the layers are electrode layers, and one or both of the electrode layers are patterned to define the pixels of the display. For example, one electrode layer may be patterned into elongate row electrodes and the other into elongate column electrodes running at right angles to the row electrodes, the pixels being defined by the intersections of the row and column electrodes. Alternatively, and more commonly, one electrode layer has the form of a single continuous electrode and the other electrode layer is patterned into a matrix of pixel electrodes, each of which defines one pixel of the display. In another type of electro-optic display, which is intended for use with a stylus, print head or similar movable electrode separate from the display, only one of the layers adjacent the electro-optic layer comprises an electrode, the layer on the opposed side of the electro-optic layer typically being a protective layer intended to prevent the movable electrode damaging the electro-optic layer.
The manufacture of a three-layer electro-optic display normally involves at least one lamination operation. For example, in several of the aforementioned MIT and E Ink patents and applications, there is described a process for manufacturing an encapsulated electrophoretic display in which an encapsulated electrophoretic medium comprising capsules in a binder is coated on to a flexible substrate comprising indium-tin-oxide (ITO) or a similar conductive coating (which acts as one electrode of the final display) on a plastic film, the capsules/binder coating being dried to form a coherent layer of the electrophoretic medium firmly adhered to the substrate. Separately, a backplane, containing an array of pixel electrodes and an appropriate arrangement of conductors to connect the pixel electrodes to drive circuitry, is prepared. To form the final display, the substrate having the capsule/binder layer thereon is laminated to the backplane using a lamination adhesive. (A very similar process can be used to prepare an electrophoretic display usable with a stylus or similar movable electrode by replacing the backplane with a simple protective layer, such as a plastic film, over which the stylus or other movable electrode can slide.) In one preferred form of such a process, the backplane is itself flexible and is prepared by printing the pixel electrodes and conductors on a plastic film or other flexible substrate. The obvious lamination technique for mass production of displays by this process is roll lamination using a lamination adhesive. Similar manufacturing techniques can be used with other types of electro-optic displays. For example, a microcell electrophoretic medium or a rotating bichromal member medium may be laminated to a backplane in substantially the same manner as an encapsulated electrophoretic medium.
As discussed in the aforementioned U.S. Pat. No. 6,982,178, (see column 3, lines 63 to column 5, line 46) many of the components used in solid electro-optic displays, and the methods used to manufacture such displays, are derived from technology used in liquid crystal displays (LCD's), which are of course also electro-optic displays, though using a liquid rather than a solid medium. For example, solid electro-optic displays may make use of an active matrix backplane comprising an array of transistors or diodes and a corresponding array of pixel electrodes, and a “continuous” front electrode (in the sense of an electrode which extends over multiple pixels and typically the whole display) on a transparent substrate, these components being essentially the same as in LCD's. However, the methods used for assembling LCD's cannot be used with solid electro-optic displays. LCD's are normally assembled by forming the backplane and front electrode on separate glass substrates, then adhesively securing these components together leaving a small aperture between them, placing the resultant assembly under vacuum, and immersing the assembly in a bath of the liquid crystal, so that the liquid crystal flows through the aperture between the backplane and the front electrode. Finally, with the liquid crystal in place, the aperture is sealed to provide the final display.
This LCD assembly process cannot readily be transferred to solid electro-optic displays. Because the electro-optic material is solid, it must be present between the backplane and the front electrode before these two integers are secured to each other. Furthermore, in contrast to a liquid crystal material, which is simply placed between the front electrode and the backplane without being attached to either, a solid electro-optic medium normally needs to be secured to both; in most cases the solid electro-optic medium is formed on the front electrode, since this is generally easier than forming the medium on the circuitry-containing backplane, and the front electrode/electro-optic medium combination is then laminated to the backplane, typically by covering the entire surface of the electro-optic medium with an adhesive and laminating under heat, pressure and possibly vacuum. Accordingly, most prior art methods for final lamination of solid electrophoretic displays are essentially batch methods in which (typically) the electro-optic medium, a lamination adhesive and a backplane are brought together immediately prior to final assembly, and it is desirable to provide methods better adapted for mass production.
The aforementioned U.S. Pat. No. 6,982,178 describes a method of assembling a solid electro-optic display (including an encapsulated electrophoretic display) which is well adapted for mass production. Essentially, this patent describes a so-called “front plane laminate” (“FPL”) which comprises, in order, a light-transmissive electrically-conductive layer; a layer of a solid electro-optic medium in electrical contact with the electrically-conductive layer; an adhesive layer; and a release sheet. Typically, the light-transmissive electrically-conductive layer will be carried on a light-transmissive substrate, which is preferably flexible, in the sense that the substrate can be manually wrapped around a drum (say) 10 inches (254 mm) in diameter without permanent deformation. The term “light-transmissive” is used in this patent and herein to mean that the layer thus designated transmits sufficient light to enable an observer, looking through that layer, to observe the change in display states of the electro-optic medium, which will normally be viewed through the electrically-conductive layer and adjacent substrate (if present); in cases where the electro-optic medium displays a change in reflectivity at non-visible wavelengths, the term “light-transmissive” should of course be interpreted to refer to transmission of the relevant non-visible wavelengths. The substrate will typically be a polymeric film, and will normally have a thickness in the range of about 1 to about 25 mil (25 to 634 μm), preferably about 2 to about 10 mil (51 to 254 μm). The electrically-conductive layer is conveniently a thin metal or metal oxide layer of, for example, aluminum or ITO, or may be a conductive polymer. Poly(ethylene terephthalate) (PET) films coated with aluminum or ITO are available commercially, for example as “aluminized Mylar” (“Mylar” is a Registered Trade Mark) from E.I. du Pont de Nemours & Company, Wilmington Del., and such commercial materials may be used with good results in the front plane laminate.
Assembly of an electro-optic display using such a front plane laminate may be effected by removing the release sheet from the front plane laminate and contacting the adhesive layer with the backplane under conditions effective to cause the adhesive layer to adhere to the backplane, thereby securing the adhesive layer, layer of electro-optic medium and electrically-conductive layer to the backplane. This process is well-adapted to mass production since the front plane laminate may be mass produced, typically using roll-to-roll coating techniques, and then cut into pieces of any size needed for use with specific backplanes.
U.S. Pat. No. 7,561,324 describes a so-called “double release sheet” which is essentially a simplified version of the front plane laminate of the aforementioned U.S. Pat. No. 6,982,178. One form of the double release sheet comprises a layer of a solid electro-optic medium sandwiched between two adhesive layers, one or both of the adhesive layers being covered by a release sheet. Another form of the double release sheet comprises a layer of a solid electro-optic medium sandwiched between two release sheets. Both forms of the double release film are intended for use in a process generally similar to the process for assembling an electro-optic display from a front plane laminate already described, but involving two separate laminations; typically, in a first lamination the double release sheet is laminated to a front electrode to form a front sub-assembly, and then in a second lamination the front sub-assembly is laminated to a backplane to form the final display, although the order of these two laminations could be reversed if desired.
U. S. Pat. No. 7,839,564 describes a so-called “inverted front plane laminate”, which is a variant of the front plane laminate described in the aforementioned U.S. Pat. No. 6,982,178. This inverted front plane laminate comprises, in order, at least one of a light-transmissive protective layer and a light-transmissive electrically-conductive layer; an adhesive layer; a layer of a solid electro-optic medium; and a release sheet. This inverted front plane laminate is used to form an electro-optic display having a layer of lamination adhesive between the electro-optic layer and the front electrode or front substrate; a second, typically thin, layer of adhesive may or may not be present between the electro-optic layer and a backplane. Such electro-optic displays can combine good resolution with good low temperature performance
In a high-resolution display, each individual pixel must be addressable without interference from the addressing of adjacent pixels (whether or not the electro-optic medium used is bistable). One way to achieve this objective is to provide an array of non-linear elements, such as transistors or diodes, wherein at least one non-linear element is associated with each pixel, to produce an active matrix display, as mentioned above. An addressing (pixel) electrode, which addresses one pixel, is connected to an appropriate voltage source through its associated non-linear element. Conventionally, in high resolution arrays, the pixels are arranged in a two-dimensional array of rows and columns, such that any specific pixel is uniquely defined by the intersection of one specified row and one specified column. The sources of all the transistors in each column are connected to a single column electrode, while the gates of all the transistors in each row are connected to a single row electrode; the assignment of sources to rows and gates to columns is conventional and could be reversed if desired. The row electrodes are connected to a row driver, which essentially ensures that at any given moment only one row is selected, i.e., that there is applied to the selected row electrode a voltage such as to ensure that all the transistors in the selected row are conductive, while there is applied to all other rows a voltage such as to ensure that all the transistors in these non-selected rows remain non-conductive. The column electrodes are connected to column drivers, which place upon the various column electrodes voltages selected to drive the pixels in the selected row to their desired optical states. (The aforementioned voltages are relative to a common front electrode which is conventionally provided on the opposed side of the electro-optic medium from the non-linear array and extends across the whole display.) After a pre-selected interval known as the “line address time” the selected row is deselected, the next row is selected, and the voltages on the column drivers are changed so that the next line of the display is written. This process is repeated so that the entire display is written in a row-by-row manner.
In the discussion below, the term “waveform” will be used to denote the entire voltage against time curve used to effect the transition of a pixel from one specific initial gray level to a specific final gray level. Typically such a waveform will comprise a plurality of waveform elements; where these elements are essentially rectangular (i.e., where a given element comprises application of a constant voltage for a period of time); the elements may be called “pulses” or “drive pulses”. The term “drive scheme” denotes a set of waveforms sufficient to effect all possible transitions between gray levels for a specific display. A display may make use of more than one drive scheme; for example, U. S. Pat. No. 7,012,600 teaches that a drive scheme may need to be modified depending upon parameters such as the temperature of the display or the time for which it has been in operation during its lifetime, and thus a display may be provided with a plurality of different drive schemes to be used at differing temperature etc. A set of drive schemes used in this manner may be referred to as “a set of related drive schemes.”
Prior art front electrodes for use with the electrophoretic and similar electro-optic displays typically comprise a very thin (about 0.1 μm) layer of a ceramic, such as indium tin oxide or a similar mixed metal oxide (see the aforementioned U.S. Pat. No. 6,982,178). This thin layer is normally formed by sputtering the ceramic on to a polymer film, typically poly(ethylene terephthalate). Prior art rear (pixel) electrodes may be formed in a similar manner, or may be formed from thin metal films; the front electrode must of course be light-transmissive to enable the electro-optic layer to be seen, whereas with a reflective electro-optic layer, the rear electrodes can be opaque.
Although ceramic front electrodes have been in large scale commercial use for a many years, they still suffer from a number of mechanical, optical and electrical problems. Ceramics are inherently brittle, and the tensions and temperatures used during lamination steps in the manufacture of the display may cause the ceramic to crack and form discontinuities in conductivity, leading to poor or inconsistent switching of the display. These cracks are also areas of high water vapor transmission, which may cause local damage to a humidity-sensitive electro-optic medium (many of the aforementioned types of electro-optic media are sensitive to humidity). In color display using color filter arrays (CFA's), it is desirable to reduce parallax problems by bringing the CFA as close as possible to the electro-optic layer using a thin polymeric film. As the polymeric film substrate is made thinner the cracking issues are exacerbated due to the higher thermal shrinkage of the thin film.
Furthermore, ceramic electrodes normally have substantially higher indices of refraction than the polymeric films on which the electrodes are deposited. For example, ITO has a refractive index at 500 nm of 1.95, whereas PET has a refractive index of only 1.65 at the same wavelength. This results in large Fresnel losses at the ceramic electrode/polymeric film interface, thus reducing the amount of light reaching the electro-optic layer. Also, many ceramic electrodes tend introduce slight color distortions; for example, ITO has higher absorption at the blue end of the visible spectrum and thus tends to impart to black-and-white displays a visually-displeasing yellow tint.
Furthermore, as discussed in detail in the aforementioned U.S. Pat. No. 7,119,772, it has been found desirable for at least some types of electro-optic display that the drive scheme at each pixel location be DC balanced, in the sense that, for any series of transitions beginning and ending at the same gray level, the algebraic sum of the impulses applied during the series of transitions be bounded. It has been found that accurately DC-balanced waveforms (i.e., those in which the integral of current against time for any particular pixel of the display is held to zero over an extended period of operation of the display) are required to preserve image stability, maintain symmetrical switching characteristics, and provide the maximum useful working lifetime in certain displays of the prior art. Waveforms which are not DC-balanced may result in polarization kickback (a change in the optical state of an electro-optic medium in a short period after the medium ceases to be driven; for example, a pixel driven to black may revert to a dark gray in a short period after the waveform concludes) and damage to the electrodes. These effects may occur at either the interface between the front electrode and an adjacent lamination adhesive layer or at the interface between a rear electrode and an adjacent lamination adhesive layer, and are primarily governed by the electrochemical properties of the materials forming the electrode and the lamination adhesive; rear electrodes may for example be formed from ITO or nickel-boron (NiB).
It is in general preferred that all individual waveforms within a drive scheme be DC balanced, but in practice this has been difficult to achieve, so typical drive schemes have usually been a mixture of DC balanced and DC imbalanced waveforms, even though the drive scheme as a whole has been DC balanced.
As discussed in the aforementioned U.S. Pat. No. 7,119,772, the extent to which DC-imbalanced driving affects an electrophoretic or other electro-optic display (presumably by polarization of certain display components, as discussed in more detail below) may be ascertained by measuring the open-circuit potential (hereinafter for convenience called the “remnant voltage”) of a particular region (say, a pixel) of the display. When the remnant voltage of a pixel is zero, it is taken to be DC balanced. If its remnant voltage is positive, it is taken to be DC unbalanced in the positive direction. If its remnant voltage is negative, it is taken to be DC unbalanced in the negative direction. Non-zero remnant voltages have been found to correlate with difficulties in accurate gray level placement.
The degradation in display performance caused by development of remnant voltage is generally reversible, either by storing the display without further switching or by switching appropriately to rebalance the DC impulses. In cases where a prior art electrophoretic display is driven with extreme degrees of DC-imbalance, however, it is possible that the electrodes may be irreversibly degraded, presumably by electrochemical reactions that consume the electrode materials.
Although DC-balanced driving waveforms effectively protect against development of remnant voltages and electrode degradation there are problems associated with their use. Extra time must be allocated in order to provide balancing impulses, sometimes resulting in update times that are two to three times longer than would be possible with a DC-imbalanced drive. In some electrophoretic compositions the time required for an optical transition from black to white is different from that required from white to black. In a DC-balanced waveform the longer of the two switching times must be used for both transitions. In addition, distracting optical transitions may be visible to the user of the display during DC-balanced updates.
Efforts have been made to reduce or eliminate the aforementioned problems in order to permit the use of non-DC-balanced drive schemes in electro-optic displays. As described for example in U.S. Pat. Nos. 6,724,519 and 7,564,614, corrosion inhibitors may be incorporated into electro-optic displays to prevent damage to electrodes from DC imbalances during driving of the displays. The aforementioned application Ser. No. 14/152,067 teaches an electro-optic display comprising at least 10 micromoles per square meter of the viewing surface of at least one compound having an oxidation potential more negative that about 150 mV with respect to a standard hydrogen electrode, as measured at pH 8. The addition of such redox materials has been shown to help prevent yellowing of ITO and the accumulation of potential differences believed to be responsible for polarization kickback.
Conduction within a ceramic electrode occurs via electron transfer whereas conduction within a lamination adhesive occurs via ion transfer. It is believed (although the invention is in no way limited by this belief) that redox couples introduce a pathway for electron transfer between the electrode and lamination adhesive, thus increasing faradic current through the double layers at the electrode/adhesive interface, and discharging the electrical double layer and lowering polarization kickback. The redox molecule typically consists of an oxidizing agent with reduction potential lower than the ceramic and this molecule is reduced preferentially instead of the ceramic, hence preventing or delaying coloration due to oxidation of the ceramic.
The present invention relates to multi-layer electrode structures which can reduce or eliminate at least some of the aforementioned problems associated with the use of ceramic electrodes in electro-optic displays.